The grinding of coal for use as a fuel in coal fired burners is usually carried out with either a bowl mill in which the grinding rollers are stationary but move radially while a bowl rotates, or a centrifugal roller mill having a fixed bull ring. The usual operation of the centrifugal roller mill is to set the rotational speed for its most efficient speed to produce maximum output, as well as to have a supply of coal in the grinding chamber to prevent metal to metal contact of the rolls on the bowl or bull ring. In some roller mills of the character seen in Williams prior U.S. Pat. No. 3,337,142 of Aug. 22, 1967 means is provided to control the position of the rollers so the speed of rotation of the rolls relative to the fixed bull ring can be increased to improve the mill output. In the prior Williams U.S. Pat. No. 4,022,387 of May 10, 1977, the rollers are set with an initial clearance relative to the stationary bull ring that is smaller than the product size. When the rolls are driven the roll pressure on the material at the bull ring is proportional to the square of the speed of rotation of the shaft carrying the rolls and only when the material quantity diminishes to less than the clearance space will the rolls reach the initial clearance setting, which is selected by resilient means thereby guarding the mill against destructive operation.